Studying processes of crystallization and cation ordering in Eu2Hf2O7

Abstract

Methods of local X ray anomalous diffraction and X ray absorption spectroscopy based on the interaction of synchrotron radiation with condensed matter have been used to study the processes of forma tion and evolution of the crystal structure in Eu 2 Hf 2 O 7 powders. It has been shown that the isothermal anneal ing of the X ray amorphous mixed hydroxide leads initially to the formation of nanocrystalline powders, which have a defect fluorite structure with a clearly pronounced nonequivalence of the local environment of the Eu 3+ and Hf 4+ cations. An increase in the temperature of heat treatment to ≥1200°C leads to the nucle ation and growth of nanodomains with a superstructural cation ordering of the pyrochlore type inside the coarse crystallites of the defect fluorite and, finally, at T ≥ 1500°C, to the complete phase transition of the flu orite-pyrochlore type in the Eu 2 Hf 2 O 7 system. In the pyrochlore Eu 2 Hf 2 O 7 structure, there was for the first time revealed a simultaneous broadening of the shape of the L 3 absorption edges of both hafnium and europium, which indicates a partial change in the symmetry of the nearest oxygen environment of both cations.

Thermodynamic Functions of Terbium Hafnate

“…Aung et al synthesized highly transparent optical ceramics [7], and Yasuhara and Ikesue investigated their magnetooptical properties [8]. Special attention has been given to the formation of nanocrystalline terbium hafnate and its conversion to normal-size crystalline ceramics of the pyrochlore structural type [5,[9][10][11]. The most interesting study of the set of the physical properties of Tb 2 Hf 2 O 7 pyrochlore in the range 1.8-300 K was made by Anand et al [12], who explored the magnetic properties, measured the molar heat capacity, and analyzed the increment in the molar heat capacity due to the interaction of the crystal field and 4f electrons of terbium hafnate (Schottky anomaly).…”

Thermal Expansion and Thermodynamic Functions of Europium Hafnate at 298–1300 K

“…Europium hafnate Eu 2 Hf 2 O 7 of pyrochlore structure type (Fd3m) crystallizes with the metal ratio Eu : Zr = 1 : 1 in the region of continuous hafnia-base solid solutions (1 -x)EuO 1.5 ⋅xHfO 2 (x ≈ 0.45-1.0), which have disordered fluorite crystal structure (Fm3m). Pyrochlores crystallize at high temperatures (1600-1900 K), where diffusion processes are activated [1]. Europium hafnate is a high-temperature oxide and does not experience structural transformations up to the temperature at which it converts to the disordered fluorite structure (~2500-2700 K) [2][3][4][5].…”

The effect of synthesis conditions on the structure of compounds formed in the Dy2O3–TiO2 system

“…Co-precipitation is preferred because titanates are formed in this case at lower temperatures [1]; furthermore, this method is suitable for studying the formation of crystalline phases and possible polymorphic transformations that take place during the synthesis [9,10]. In the case of polymorphism, the type of the crystal structure formed is largely determined by synthesis conditions [11].…”